1. Field of the Invention
The present invention relates to a method for manufacturing a thin film transistor. Further, the present invention relates to a display device including the thin film transistor. Further, the present invention relates to an electro-optic device typified by a liquid-crystal display device and an electronic device including the display device as a component.
In this specification, a semiconductor device means any device which can function by utilizing semiconductor characteristics; a display device, a semiconductor circuit, an electronic device are all included in the category of the semiconductor device.
2. Description of the Related Art
In recent years, a technique for forming thin film transistors (TFTs) using a semiconductor thin film (with a thickness of several tens to several hundreds of nanometers) formed over a substrate having an insulating surface has been attracting attention. Thin film transistors have been widely applied to electronic devices such as ICs or display devices. In particular, of them, development as a switching element of a display device typified by a liquid-crystal display device has been quickened.
In display devices such as liquid-crystal display devices, amorphous silicon thin film transistors have been mainly in practical use as switching elements. Typically, channel-etched inversed staggered (or bottom-gate) TFTs including a channel formation region made of amorphous silicon have been used in many cases.
In the case of the channel-etched TFT, a back-channel portion, which is an interface between amorphous silicon and an interlayer insulating film, is exposed to the air and work environment during the manufacturing of the thin film transistor, so that the back-channel portion might be contaminated and impurities might be attached to the interface to affect the off-state current. In addition, in the case of the channel-etched TFT, the back-channel portion is etched so that the interface is damaged, which causes increase of the off-state current of transistor characteristics.
Japanese Published Patent Application No. H11-274504 has disclosed a technology for removing such impurities attached to the surface of the back-channel portion, in which surface treatment is carried out with plasma discharged using an atmosphere gas containing H2 or He to remove the impurities attached to the surface of the back-channel portion.